1. Field of the Invention
The present invention relates to aromatic polyimides containing the hexafluoroisopropylidene group or the 1-phenyl-2,2,2-trifluoroethane group and having an intermediate molecular weight, and to a method for their preparation.
2. Description of Related Art
Polyimides in general are well known in the art to be useful for high temperature applications, since they have a glass transition temperature of about 300 degrees Celsius and above. Such polymers may be prepared in any number of ways, perhaps the most common method being a two-step process including reacting a dianhydride such as pyromellitic dianhydride (PMDA) with a diamine to form a soluble polyamic acid which is then cyclized, thermally or by chemical means to form a polyimide.
Such procedures have been employed in connection with the preparation of fluorinated polyimides as shown, for example, in U.S. Pat. No. 3,356,648 to Rogers. Example 11 of the '648 patent discloses a method of preparing a polyimide from 2,2-bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride and 2,2-bis(4-aminophenyl) hexafluoro- propane. Equimolar amounts of the diamine and dianhydride are stirred together in dioxane for about eighteen hours at room temperature to form a polyamic acid. To the polyamic acid is added acetic anhydride and a minor amount of beta-picoline. After stirring for about 15 minutes, without cooling, the mixture is poured onto a glass plate to form a gel film. The gel film is heated in an oven at 120.degree. C. for twelve hours, then heated two more hours at 250.degree. C. to form a polyimide film. The polyimide film thus produced is reportedly soluble in chloroform, benzene, dioxane and acetone.
Other fluorinated polyimides are disclosed in U.S. Pat. No. 3,959,350 to Rogers. In Example I of the '350 patent, a fluorinated polyimide is prepared by mixing equimolar amounts of the 2,2-bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride and 4,4'-diaminodiphenyl ether in dimethylacetamide under a nitrogen atmosphere at room temperature. The intermediate polyamic acid is converted to the corresponding polyimide by adding beta-picoline and acetic anhydride.
Fluorinated polyimides prepared as above do not have the desired properties in terms of molecular weight, color and other parameters required for many applications and thus further work has been done in this field. For instance, in U.S. Pat. No. 4,645,824 to Landis et al., there is disclosed and claimed a method of preparing high molecular weight fluorinated polyimides prepared by way of cresol solution. In the '824 patent, a method of preparing polyimides is described yielding polymers with molecular weights of up to about 35,000.
In copending U.S. application Ser. No. 217,929, filed in the USPTO on July 12, 1988, a process is described for synthesizing an ultra high molecular weight polyimide based on the condensation product of 2,2'-bis(3,4-di- carboxyphenyl) hexafluoro- propane dianhydride and 2,2'-bis(4-aminophenyl) hexafluoropropane or 2,2'-bis- (3-aminophenyl) hexafluoropropane. While these polymers exhibit superior mechanical, electrical and optical properties, they, like the polymers described in the aforementioned patents, have certain limitations particularly in applications relating to the microelectronics field.
A particularly desirable application for polyimide polymers is in microelectronic applications where polymer films having superior thermooxidative stability and good insulating properties are required. Such applications include dielectric interlayers and passivation coatings used in the production of electronic circuitry chips and semiconductor devices, including applications such as disclosed for example in U.S. Pat. No. 4,692,205. Such devices are normally prepared by forming a coating of a solution of polyimide on the surface of a silicon wafer by spin coating techniques, followed by the application of other photosensitive or processing layers. The method for processing such chips involves subjecting them to extremely high temperatures (at least 200.degree. C.) as required for the etching and/or baking processes involved in the fabrication of such chips.
Ideally, the dry polyimide film which is spin coated onto the underlying substrate should be of uniform thickness, usually within the range of from about 25 to about 90 microns, preferably from about 35 to about 60 microns. The achievement of such uniform film thicknesses at given spin application rotation speeds is largely a function of the weight average molecular weight of the polyimide. Where relatively low molecular weight polymers are used, e.g., below about 50,000, the viscosity of the polymer solution employed in the spin coating process is very low such that films of uniform thickness above about 25 microns are difficult to achieve. The resulting thin films are relatively brittle and exhibit inferior mechanical and insulating properties. Where relatively high molecular weight polymers are used, e.g., above about 150,000, the viscosity of the coating solution will be very high and the solubility of the polymer relatively low. In such cases, the polymer solution will not flow uniformly on the surface of the silicone wafer giving rise to a striation-like uneven surface. When such a coated wafer is subjected to heat treatment, a stress is built up which may cause the film to lift from the surface of the wafer upon cooling.
In order to readily avoid the aforementioned pitfalls, it is most desirable that the average molecular weight (Mw) of the polyimide polymer be within the relatively narrow range of about 80,000 to about 135,000 in order to more readily produce a controllable dried coating thicknesses within the range of about 25 to about 90 microns, or more preferably within the range of about 35 to 60 microns.
To synthesize such a narrow molecular weight polymer, one could use the standard method described in the literature, such as end-capping the polymer chain to prevent further molecular weight build-up, as disclosed in U.S. Pat. No. 3,234,181. The end-capping compounds traditionally used are phthalic anhydride or aniline for condensation type polyimides. Controlled molecular weight polymers may be achieved by this technique, but at the expense of their thermal properties. End-capped polyimide polymers degrade rapidly at elevated temperatures during the stepwise baking process. Also, due to instability of the end-capping agent to moisture, the possibility of gelation of the coating solution over a period of time may exist. Coated films may also develop a dark amber color due to oxidative instability of the end-capping agents at elevated temperatures in air. These are a few of the many disadvantages of the end-capped polymer.